Precious stones, rare stones

The sparkling sparkle of a diamond, the bluish frost of a Siberian topaz, or on the contrary the deep blue of a tanzanite… “Precious” stones are a delight for the eyes. Officially, the denominations “precious” and “semi-precious”, or “semi-fine”, have ended since a decree of Lionel Jospin in 2002. In reality, whatever the name, the beauty of these gems reveals their character. exceptional.

“Except that the beauty, and therefore the precious side of these stones, is very subjective and cultural, recalls François Farges, professor at the National Museum of Natural History, specializing in mineralogy. If the diamond is highly valued in the West for its shards, it was not in China, where the elites preferred imperial jade. “ In recent decades, diamonds have become more popular with Asians, possibly due to cultural globalization. As one jeweler sums it up: “The ideal gemstone is one that is known to everyone but that only a handful of customers can afford. “

When it comes to stone, everything is a matter of taste and color. Even if the latter can be downright misleading! A ruby ​​and a sapphire, for example, come from the same pebble without much interest, corundum. “99% of corundum in nature has inclusions and therefore a rather brown and opaque color, says François Farges. On a path, no one will bend down to pick them up. On the other hand, a sapphire or a ruby ​​…

Precious binoculars owe their color to additional chemical elements. But where the chromium that comes in a corundum will give the red tint of ruby, the same chromium in a beryl – another kind of mineral – will give… the green of the emerald! “The same chemical element does not always give the same color depending on the crystal lattice, confirms the specialist. The structure on which the element is grafted counts. “ The training conditions too.

The three “major” rock types (magmatic, sedimentary and metamorphic) each contain specific gemstones. “The training conditions often play more than the chemical composition”, argues Grégoire Michel, president of the French Society of Mineralogy and member of the Géosciences Environnement Toulouse laboratory. Take the diamond, for example. Chemically, it is made up of carbon, an element that is very common on Earth.

But this carbon is confronted with strong heat and high pressures: temperatures of at least 1,100 degrees to more than 150 kilometers under the earth’s crust. “The diamonds formed must then be brought up very quickly by violent magmatic eruptions, describes Grégoire Michel. If the ascent is too slow, the diamond turns back into graphite. “ And, instead of a gem, Earth provides a pencil lead.

This is where the ambiguity of “precious” stones lies: their elements are not, in themselves, rare, but it only takes a little for the gem to become a common pebble. Hard to believe, but for example between the brick red of bauxite pebbles and the brilliant carmine of rubies, it is still aluminum oxide. “Except that in one case we make casseroles and in the other jewelry!” “, smiles Gaston Giuliani, from the Petrographic and Geochemical Research Center (University of Lorraine-CNRS). Another example is quartz, which in one case gives a beautiful amethyst and, in the other, a funerary granite.

Training conditions also influence the availability of gems. Where there are hundreds of amethyst deposits – if not more! -, there is for example only one deposit of tanzanite, in the country which gives it its name, and about fifteen deposits of jade exploited commercially. The geological origin is very important for colored stones, supports François Farges. For example, a sapphire from Kashmir is very valuable because the deposits have only been mined there for thirty years. On the other hand, a sapphire from Sri Lanka has less value because these mines produce a lot and the color is less rich.

“We must also take into account the final cost of operation, recalls Gaston Giuliani. Is it difficult to get the gems out of their rocks? “ Diamond, for example, is not rare geologically. The proof, it is even made of abrasive powders and polishing wheels, because of its hardness. “But on a ton of rock, on average, only five grams of diamond are produced, and only 20% of these diamonds prove to be exploitable in gem quality., details the expert. Even though there are many producing countries and many deposits, the quantity of diamonds produced does not meet the demand for gems.. As François Farges reminds us, “Humans will never be able to extract more than 0.01% of the precious stones present in the Earth “.

In the future, the most valuable may not be the ones we imagine. The screens, batteries, and other high-tech objects that inhabit everyday life consume a large quantity of so-called “critical” or “rare” metals. “It’s really their chemical elements that are rare and coveted, indicates Bénédicte Cenki, from the Geosciences laboratory at the University of Montpellier. For these critical metals, the order of magnitude would amount to seeking a family in a town of million inhabitants.

In Europe, only 3% of needs are covered by internal production, everything else must be imported. If Brazil is a major supplier of precious stones, it is China that holds the podium. And dependence and the risks weighing on the supply do not have the same consequences at all ”, alert the specialist. Without sapphires and emeralds would not upset our daily life. But a disruption in the supply of cobalt from batteries, or indium from LCD screens, would require a review of all habits.


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